TECTONO-STRATIGRAPHIC SETTING OF THE MORETON'S HARBOUR GROUP AND ITS IMPLICATIONS FOR THE EVOLUTION OF THE LAURENTIAN MARGIN

Characterization of age and environment of formation of the composite Laurentian margin along the Red Indian Line places important constraints on the geometry of the Laurentian margin prior to closure of the Cambro-Ordovician Iapetus Ocean and collision with peri-Gondwanan terranes. The Moreton’s Harbour Group is located in proximity to the Red Indian Line and marks the north-western most extent of the peri-Laurentian Annieopsquotch Accretionary Tract. It comprises pillow basalt, sheeted diabase, gabbro and layered gabbro with minor trondhjemite, rhyolitic tuff and chert; this assemblage is typical of an incomplete ophiolite sequence. The ophiolite is dissected by syn-magmatic intra-oceanic high angle shear zones that offset the stratigraphy locally juxtaposing felsic tuff and layered gabbro. These faults are spatially and temporally associated with a suite of syn-tectonic trondhjemite dykes. A sample of trondhjemite has yielded a U/Pb ID-TIMS zircon age of 477.4±0.8 Ma, constraining the age of ophiolite formation and intra-oceanic extensional deformation. Moreton’s Harbour Group basalt and diabase display enriched mid-oceanic ridge basalt to island arc tholeiite chemistry consistent with their derivation in a supra-subduction zone setting such as a backarc or rifted arc.

The paleogeographic location of the Moreton’s Harbour Group has been previously constrained to be near the Laurentian margin (11°S), hence the Moreton’s Harbour Group forms an important element in the reconstruction of the paleogeography of the Cambro-Ordovician Iapetus Ocean. The tectonic setting and age of the Moreton’s Harbour Group is consistent with c. 480 Ma Annieopsquotch ophiolite belt and c. 478 Ma Mansfield Cove Complex that record subduction zone initiation and development of a nascent arc outboard of the Dashwoods microcontinent and its Notre Dame Arc supra-structure. The correlation of the Moreton’s Harbour Group with the Annieopsquotch ophiolite belt confirms previous hypothesis that the Annieopsquotch ophiolite belt formed in proximity to Dashwoods following subduction flip.